Method for manufacturing an element formed by semiconductor(s) and gas detector provided with such a semiconductor

ABSTRACT

Process for manufacturing a semiconductor element, particularly for a flue gas detector, wherein two or more different substances, at least one of which is a semiconductor in insoluble powder form, are dispersed in powder form in a solvent, and the heterogenous suspension of the semiconductor material thus obtained is applied in granular form to an insulating substrate with a specific resistivity value of at least 10 12  Ω cm.

The present invention is directed to a method for manufacturing anelement formed by semiconductor(s) to be used for detecting a gasproduced by combustion, according to which at least two substances ofdifferent composition, of which at least one is a semiconductor in apowder state, are dispersed in the state of powders insoluble in asolvent to obtain a heterogeneous suspension wherein the granular stateof the semiconductor substance is maintained. The heterogenoussuspension is applied on an insulating substrate having a specificresistivity value of at least 10¹² Ω cm, said solvent being eliminatedafter application of the suspension.

Such a method is known from the U.S. Pat. No. 4,381,922. According tothe known method, a mixture of semiconductor powders composed ofmetallophthalocyanines which are organic semiconductors is prepared.These powders are poured into an organic solvent in order to modifytheir initial phthalocyanine structure. In order to form the elementformed by semiconductor(s), the suspension thus obtained is applied onan insolating substrate provided with the necessary electrical contacts.

Such elements formed by semiconductors are used in combustion gasdetectors, such as for example fire detectors or exhaust gas detectors.According to the particular choice of the type of semiconductor or ofthe organic semiconductor substance used, they can be renderedparticularly qualified for a well determined application.

A drawback of the known semiconductors is that they present a molecularcrystalline structure and that the intermolecular bonds are assured byvan der Waals forces. In these circumstances, with time and even atambient temperature, bonds can be established between the constitutivepowder particles of the semiconductor powder used. This results in anatural and progressive sintering of the preparations. Over time, thisphenomenon leads to a quite rapid decrease in the semiconductor'sspecific surface which implies a limited life span of the detectorsprovided with such an element formed by semiconductor(s). Indeed, thepowder is dissolved in the organic solvent in order to enable theinitial phthalocyanine molecular structure to be modified and to obtaina homogeneous suspension. The organic character of the deposited powdersthus leads to a sintering of the phthalocyanine powder particles whichprovokes a decrease of the specific surface at relatively short-term.The sintering phenomenon provokes, at ambient temperature, a sticking ofpowders which highly reduces the sensibility to the gaseous agents ofthe element formed by semiconductor(s).

The object of the invention is to realise a method for manufacturing anelement composed of semiconductor(s) destined to a detector for gasproduced by combustion having a longer life span while not limiting itsapplications.

To this end, a method for manufacturing an element composed ofsemiconductor(s) according to the invention is characterised in that thepowders are not submitted to sintering and in that at least onesemiconductor substance is a mineral semiconductor. The powders nolonger being submitted to sintering neither during the manufacturing ofthe semiconductor nor in time, a substantially greater specific surfaceis obtained. The nature and the structure of the semiconductor barelychange any more. Since different substances are used, the sinteringprocess caused by time appears only feebly, what sensibly lengthens thelife span of the semiconductor thus realised. Moreover, the mineral ororganic semiconductors are generally in the state of powders depositedwithout sintering, what renders them totally suitable for the methodaccording to the invention and enables a detection at ambienttemperature. In thus avoiding the emergence of a sintering process, thesensibility of the captors no longer sensibly evolves over time.

A first preferred embodiment of a method according to the invention ischaracterised in that a mineral semiconductor of one of the n or p typesis used which leads to n-p-n or p-n-p junctions by coupling with anothermineral or organic semiconductor. An element formed by semiconductor(s)is thus obtained of which the characteristics can be modified infunction of the choice of the type of semiconductors and theirassociation. The particular use to which the detector is destined willdetermine the choice of the weighting of the semiconductors of type n,type p to be used.

Preferably, the semiconductor substances are chosen amongst tin, indium,cobalt, copper, antimony, germanium, gallium, nickel, chrome, zinc ortitanium oxides. These oxides can be found at will on the market in thestate of powders, what renders them particularly suitable for theapplication of the method according to the invention. Moreover, sincethere is no sintering these elements formed by semiconductors can beused at ambient temperature to detect a combustion emitted gas.

A second preferred embodiment of a method according to the invention ischaracterized in that a powder formed by an inert compound selectedfrom, amongst others, alumina of silica is also dispersed in saidsolvent. The inert compound does not modify, on one hand, the nature andthe structure of the semiconductor and, on the other hand, itcontributes to maintain a great specific surface which limits thesintering effect in time. A mixture of a semiconductor in the powderstate and of an inert component thus perfectly conforms with the conceptof the present invention.

Preferably, before being dispersed the size grading of the semiconductorsubstances is reduced by grinding to less than 100 μm. This enables toobtain a certain uniformity in the dimensions of the powder particlesthat consequently avoids the formation of agglomerates in the mixturesand thus enables to obtain a great specific surface of thesemiconductor.

A third preferred embodiment of a method according to the invention ischaracterised in that said suspension is applied in successive layers onthe substrate, each applied layer being followed by a drying to evacuatethe solvent. Several layers are thus obtained on a same substrate, whichis all benefit for obtaining a great specific surface.

The invention also relates to a gas detector comprising an elementformed by semiconductor(s) obtained by application of the abovementioned method. Such a detector is serially mounted with an adjustmentmodule and is connected to a verification unit provided for detecting afast variation in the resistive value of the element formed bysemiconductor(s) and to generate an output signal after detection ofsuch a variation. The verification unit operates the selection betweenslow and fast variation by control, after a certain delay, of theadjustment module by varying its impedance in such a manner that thevoltage at the terminals of the element formed by semiconductor(s)remains sensibly constant upon feeble variations and this whatever theimpedance of this element may be. Thus, only the relative value of theimpedance of this element, and not its absolute value, intervenes in thedetection. The detection of a fast variation enables to rapidly detect achange of the semiconductivity and thus the presence of a gas producedby combustion, which renders the detector highly performant.

Preferably, said adjustment module comprises a transistor and acapacitor connected in parallel, said adjustment signals being suppliedat the base of said transistor. This offers a solution easilyintegrable.

The invention will now be described into more detail by means of thedrawings amongst others. In the drawings:

FIG. 1 shows, at an enlarged scale, a semiconductor element obtained byapplication of the method according to the invention.

FIGS. 2, respectively 4 and 5 show the change in resistance in functionof time due to the presence of a fire with flames, of a semiconductorelement, according to the invention and comprising copperphthalocyanine, respectively tin oxide and indium oxide.

FIG. 3 shows the change in resistance in function of time due to thepresence of a fire without flames of a semiconductor element, accordingto the invention and comprising copper phthalocyanine.

FIG. 6 shows the sensibility (ρ) of a semiconductor element according tothe invention.

FIG. 7 shows an example of embodiment of a detector according to theinvention.

Amongst the semiconductors, semiconductors of the type n and the type pare distinguished. The properties of these semiconductors, calledintrinsic semiconductors, are generally determined by the compositionand the structure of the metallic oxide forming the semiconductorsubstance.

An extrinsic semiconductivity can however occur and in that case, it canbe determined by the presence of certain gases in the atmosphere.According to the nature of the gas, the latter acts as a dopant byincreasing or decreasing the number of charge carriers.

According to the presently known methods, one or more metallic oxidesforming the semiconductors are used either in thin films, or in thestate of powder having undergone a sintering at high temperature (about800° C.). In consideration of the feeble value of the semiconductivityof these oxide layers and the feeble specific surfaces obtained by thesetechniques, the detectors thus obtained can only be used at hightemperature, in a range of 450° to 600° C., in order to obtain the mostadequate sensibility with respect to the application aimed at. Thesedetectors are thus used for the detection of combustion gases such asfor example H₂, CO, CH₄ and other carbon hydrides.

In the case where a mixture of powders is realised, for example bariumoxide and titan oxide, and where these powders are submitted to asintering operation at high temperature, two phenomena occur:

1/ A reaction between the barium oxide and the titan oxide which leadsto the formation of barium titanate;

2/ A bond of the powder particles together by interdiffusion which leadsto the formation of powder particles of larger dimension.

In this latter case, the active surface is no longer equal to thegeometrical surface but corresponds to the one of the specific surfaceof the new formed structure, in this case that of the barium titanate.

However this specific surface is distinctly inferior to the onepresented in the beginning by the used powders since they have undergonea sintering and have been bonded together by interpenetration of theircrystalline network.

Another technique used to manufacture a semiconductor element isevaporation in vacuum. By this technique, a semiconductor film isobtained of which the active surface strictly corresponds to theapparent surface. This signifies that if for example, a tin oxide filmis deposited, the film obtained presents an active surface strictlyequal to the one of the geometrical surface.

To detect gases produced by a combustion organic semiconductors of thefamily of the porphyrins such as for example tetrabenzoporphyrin andmetallic phthalocyanine are generally used. In most cases, it is aextrinsic semiconductivity since it is conditioned by the donor oracceptor character of the gas molecules present in the atmosphere.

However it should be noted that none of the semiconductors of mineralnature seem to have been used at ambient temperature for the detectionof gas present in the atmosphere. The literature even describes thatthey may not be used in these conditions. The reason of this rejectionis surely inherent to the production techniques and to the electronicprocessing of the information supplied by the detector.

The use of only one semiconductor substance to form a semiconductorelement without the presence of other substances also creates a problemindeed, in time the semiconductor substance can undergo a progressivesticking. The sticking together of the powder particles provokes adecrease of the active surface and thus of the sensibility of thedetector.

In order to enable the use of the detector, provided with asemiconductor element, at ambient temperature and to avoid the sinteringprocess, the present invention proposes a manufacturing method of asemiconductor element wherein at least two different substances in thestate of insoluble powder are used. At least one of these substancesmust imperatively be a semiconductor while the other can be composed ofan inert compound or another semiconductor different from the first. Thepowders are not submitted to any sintering precisely in order to avoidmodifying the nature and the structure of the semiconductor substances.

The used powders are preferably ground before applying them on aninsulating substrate having a specific resistivity value is of at least10¹² Ω cm and whereof the resistance does not evolve with the ambientfactors. The powders are ground in order to obtain a size grading isless than 100 μm. The grinding enables amongst others the dimensions ofthe powder particles to be the closest possible to one another.

The powders are then dispersed in a solvent, such as for example water,ethanol, acetone or a mixture of these solvents. Preferably thepowder-solvent suspension is submitted to an intense agitation in orderto realise a heterogeneous suspension where the different substances arewell mixed and randomly distributed in the solvent. A preferentialsegregation or sedimentation is thus avoided. Moreover this enables toconsiderably reduce the probability that two powder particles of a samesubstance subsequently deposit next to one another on the substrate.

The suspension thus obtained is then deposited on the substrate 1 insuch a manner that it is the powder particles 2 themselves, that aredeposited, such as illustrated in FIG. 1. The deposit itself occurs forexample by serigraphy, painting, electrophoresis or by simple immersionof the substrate in the suspension. Preferably the suspension is appliedin successive layers on the substrate, each applied layer being followedby a drying in order to evacuate the solvent. The drying is for examplerealised by means of hot air and enables to obtain a better adherence ofthe deposited powder.

The adherence of the layers is obtained by insertion of the powders inthe cavities due to the rugosity of the substrate used and by a form ofsticking together of the powders. The used substrates are for examplecomposed of plaquettes of sintered alumina or of oxidized silicon.

In order to enable the connection of the semiconductor element thusobtained to an electrical voltage source two electrodes are deposited onthe insulating substrate, for example by serigraphy in thick layer.These electrodes are for example obtained from paste constituted eitherby silver-palladium alloys, either by gold or another nobel metal, whichoffers the advantage of avoiding corrosion phenomena or subsequentdoping of the semiconductor powders.

In the case where the quality of the adherence of the powders on thecarriers should be improved, they can be covered by an inert compound,insulating and porous such as, for example, a layer of plaster, ofzeolite, or a porous membrane such as collodion (mixture of tetranitroand trinitro cellulose) or yet a foam composed of an organic polymerwith open porosity.

The composition, the size grading and the nature of the componentsintervening in the mixture of the sensible layer is very important. Itconditions, indeed, the base resistance of the captors, the nature oftheir response, their sensibility and their life span.

The base resistance of the captors depends on the resistance of each ofthe components taken separately and on their mutual interactions.

Thus, captors realised with the pure components present the followingresistances:

    ______________________________________                                        Copper phthalocyanine                                                                              10.sup.9 to 10.sup.10 Ω                            Indium oxide In.sub.2 O.sub.3                                                                      approx. 4 · 10.sup.4 Ω                    Tin dioxide SnO.sub.2                                                                              approx. 10.sup.8 Ω                                 Silica SiO.sub.2     insulating                                               Alumina Al.sub.2 O.sub.3                                                                           insulating                                               ______________________________________                                    

The value of the base resistance conditions the importance of theresponse obtained by the detector provided with the semiconductorelement. More the resistance is great, more the intensity of thevariation (ΔR) of the obtained signal will be great.

The nature of the response is directly conditioned by thesemiconductivity of the used powders. Thus, a captor realised withphthalocyanine powder or with another semiconductor of the p type, seesits electrical resistance decrease upon combustion with flames, as shownin FIG. 2 where the change in the resistance R is illustrated infunction of the time t1 (expressed in minutes).

In this case it is a heavy combustion where the gases produced arecompletely oxidized and thus present a very important electron acceptorcharacter towards the captor.

This acceptor character provokes an increase in the number of positivecharge carriers upon absorption, which increases the semiconductivity ofthe p type and consequently decreases the resistance.

Inversely, upon a combustion without flame provoked for example by apiece of cardboard disposed on a plate heated to about 500° C., theresistance of such a captor increases very strongly, as shown in FIG. 3.Indeed, during such a combustion, the gases emitted are not completelyoxidized and keep an electron donor character in respect tophthalocyanine. This donor character leads upon absorption to a greatdecrease of the number of positive charge carriers characteristic of aphthalocyanine of the p type. This results in a great increase in theresistance measured at the terminals of the captor.

When the semiconductor element of the n type is manufactured by using ametallic oxide such as for example tin oxide (SnO₂), indium oxide(InO₃), cobalt oxide (CO₂ O₃), antimony oxide (Sb₂ O₃), germanium oxide(GeO₂), gallium oxide (Ga₂ O₃), tantalum oxide (Ta₂ O₃), iron oxide (Fe₂O₃), tungsten oxide (WO₃), zinc oxide (ZnO) or titan oxide (TiO₂) thebehaviour is completely inverted.

Since these powders present a semiconductivity of the n type thepresence of an acceptor gas provokes a decrease in the number ofnegative charge carriers and thus an increase in the resistance of thecaptor.

This type of behaviour can be observed in the FIGS. 4 (SnO₂) and 5 (In₂O₃) upon heavy combustion, such as for example paper with flames. Whenthese two detectors are submitted to the same fire box, it appears thatthe global resistance of the captors greatly increases upon combustion.

However it should be noted that the resistance of the detector realisedwith the tin oxide powder (FIG. 4) started by decreasing beforeincreasing.

This type of behaviour clearly shows that the Fermi level is unique foreach semiconductor powder. Consequently, a given gas produced in anoxidation chain and presenting a donor behaviour with for example tinoxide can inversely present an acceptor behaviour for a captor formed bytitan oxide.

This can be explained by the fact that the gases produced uponcombustion of a compound as simple as methane undergo a series ofintermediary stages.

Thus, in this case we obtain: ##STR1##

Whether the combustion is heavy or moderate, the oxidation is completeor stops at one of the intermediary stages.

This results in that depending on the Fermi level of the semiconductorelement used, one of these gases, for example in the chosen caseformaldehyde CH₂ O can manifest a donor character or not.

This signifies that, in the preceding example, the gases have alwayspresented an acceptor character towards indium oxide, but not towardstin oxide which has presented a donor character during a few instants.

This fact is extremely important because the mixture of these powdersthat applies during the method according to the invention, willprecisely enable to determine the level at which the detector will reactwith a donor or an acceptor character. According to the applicationaimed at, the choice and the weighting of the semiconductor powders usedare adapted in order to obtain the desired response.

Thus, certain detectors destined, for example, for fire detection willbe composed of a mixture of semiconductor powders of the n type, but ifthe action of certain agents is to be decreased some semiconductor ofthe p type can de added. Such adjunctions can be justified notably toavoid false alarms due for example to the presence in the atmosphere ofa gas such as ammoniac which is found in certain cleaning products.

Moreover, the mixture of semiconductor powders of the n type and of thep type creates junctions p-n, n-p-n and p-n-p of the powder particlesupon contact which greatly modifies the responses obtained as well inthe importance of the responses than in their nature.

The life span of the captors essentially depends on the keeping in timeof the specific surface of the used powders. Indeed, in time, the slightsticking effect that exists between the powder particles can progress bya slight interdiffusion due either to the presence of two powderparticles of the same nature side by side, either to the action of ahighly reactive gas (NO_(x), SO₃, . . . ) either to the action of thevoltage imposed to its terminals. This more important sticking modifiesthe specific surface of the powders and thus decreases the importance oftheir response. This is solved by increasing to a maximum the number ofdifferent substances present in the semiconductor element. Indeed, whenthe neighbouring powder particles are not of the same nature, the riskof seeing the sticking progress greatly decreases.

The heterogeneous character of the applied suspension on the substrateaccording to the invention also contributes to sensibly decrease thissintering effect over time. The use of powder particles of differentsubstances reduces the interaction between these substances. For thissame reason, powders constituted of inert material on the chemical andelectrical plane such as silica, alumina or talc are also introduced inthe mixture. The introduction of these inert powders in the mixturekeeps the sticking from progressing month after month and allows tomaintain the sensibility (ρ) of the captors, as illustrated in FIG. 6where the time (t) is expressed in days. These inert powders have anelectrical insulating character and consequently their introduction inthe mixture increases the electrical resistance to the terminals of thecaptor. Again, the proportions and the size grading of the powders usedmust be chosen with care.

The detectors comprising a semiconductor element according to theinvention can be used in different applications where the suddenappearance of a particular gaseous constituent in the atmosphere has tobe detected. This is notably the case within the framework of firedetection because these detectors allow the detection of every type ofnormalised fires.

Since the detectors function at ambient temperature, it is not necessaryto heat them, which considerably reduces their energy consumption, sincethe power of the set-up is of approximately 0.2 mW.

With respect to the use of detectors that would only use onesemiconductor of the p type, the contribution is also very important,since the latter could not be used to answer the standards in force.Indeed, the combustion chambers used by the organisations thateffectuate these standardisations have already been used to effectuate avery great number of fires. Consequently these chambers are covered bysoot and thus strongly smell smoke. This odour is due to gas emanationsthat correspond to cold smoke and thus to gas reducers.

The semiconductors of the p type (for example phthalocyanine) present aresistance that highly increases in presence of such gases. Thisincrease is so important (R>10¹¹ Ω) that the usual electronics are nolonger capable to face it and that they directly switch over to alarmupon introduction in the chamber.

The situation is completely different with a semiconductor element ofthe n type according to the invention, since the resistances decreasewhen they are introduced therein and consequently they are even moreeasily measurable.

Analogous captors and the electronics described are also suitable forthe detection of combustion gases or certain noxious gases that appearin the atmosphere for example upon pollution.

That is the reason why they perfectly suit for the automatic regulationof ventilations in general. Amongst them, can be notably cited:

the regulation of outside air admission in the passenger spaces ofautomobile vehicles,

the regulation of the ventilation in toilets,

the regulation of the ventilation in parkings and tunnels,

the detection of noxious gas leaks such as chlorine Cl₂, hydrochloricacid HCl, cyanohydric acid HCN, sulphydric acid H₂ S, azote oxidesNO_(x), sulphur oxides SO_(x), ammonia NH₃, organic acids (formic,acetic, etc. . . . ), etc. . . .

The fact that they are susceptible of detecting the presence of gasessuch as trimethylamine would also enable to use them for example toindicate the state of freshness of fish. Indeed, in this case,emanations of trimethylamine are detected.

Also, their sensibility to sulphydric acid H₂ S, shows that they candetect the presence of --SH groupments, i.e. of mercaptans. They couldconsequently be notably used for the detection of the presence oftruffles.

Finally, the fact that these detectors present different responsesaccording to the nature of the combustion gases enables to use them forthe regulation of burners or internal combustion motors.

FIG. 7 shows an example of an embodiment of a detector according to theinvention. The detector comprises the semiconductor element 3,represented by a resistance, serially mounted with an adjustment module7. The connection between the element 3 and the module 7 is connected toa verification and processing unit 4 whereof the input is connected to areference signal generator 5. An output of the unit 4 is connected via adelay element 6 to a control input of the adjustment module 7.

The adjustment module 7 is intended to compensate the feeble variationsin time of the resistance of the semiconductor element 3. On the otherhand, a rapidly varying resistance of the semiconductor element isdetected by the verification unit 4 and provokes the generation of anoutput signal presented at the output 8 of the unit. The adjustmentmodule 7 is, for example, composed of a variable resistance, a bipolartransistor, FET or another impedance regulating unit, in order tomaintain the voltage relatively constant at the terminals of thesemiconductor element 3.

When the voltage at the terminals of the semiconductor element 3decreases or increases slightly due to a change in the semiconductorelement's resistance, this change is detected by the verfication unit 4.The verification unit 4 will produce a first or a second adjustmentsignal that will be presented to the adjustment module 7. Under controlof the first or a second adjustment signal the impedance of theadjustment module 7 will be increased or decreased in order to maintainthe sum of the impedance formed by the element 3 and the module 7substantially constant. The adjustment signal is transmitted with adelay T to the module 7, imposed by the delay element 6.

The verification and control unit can be formed by an operationalamplifier of which an input is connected to a reference voltage sourceand the output to a resistance bridge, itself connected to a circuitcomprising a charge resistance and a discharge resistance each connectedto a diode. According to an alternative embodiment the resistance bridgeis connected to two reversed biased diodes, that used in their zone atconstant voltage enables a charge and discharge of a capacitor.

Preferably, the adjustment module is formed by a transistor and acapacitor connected in parallel with the emitter and the collector ofthe transistor.

The verification and control unit can also be equipped with amicroprocessor provided to produce said adjustment signals. An analog todigital conversion unit is then connected between the semiconductorelement and the microprocessor.

I claim:
 1. A method for manufacturing an element formed bysemiconductor(s) for detecting a gas produced by combustioncomprising:(a) dispersing a first semiconductor metal oxide substanceand a second semiconductor organic substance in a solvent to obtain aheterogeneous suspension where said first and second semiconductorsubstances are dispersed in a powder state insoluble in said solventwithout being submitted to sintering; (b) applying said heterogeneoussuspension, wherein the granular state of said substances is maintained,on an insulating substrate having a specific resistivity value of atleast 10¹² Ω cm; and (c) removing said solvent.
 2. The method formanufacturing an element formed by semiconductor(s) according to claim1, wherein said first semiconductor formed by said semiconductor metaloxide is of the n and p type in order to form n-p-n or p-n-p junctions.3. The method for manufacturing an element formed by semiconductor(s)according to claim 1 or 2, wherein said first semiconductor is a metaloxide wherein the metal is selected from the group consisting of tin,indium, cobalt, copper, antimony, germanium, gallium, nickel, chrome,zinc and titanium, and said second semiconductor substance is aporphyrin.
 4. The method according to claim 3, wherein said porphyrin isa phthalocyanine.
 5. The method for manufacturing an element formed bysemiconductor(s) according to claim 1 or 2, further comprisingdispersing in said solvent, a semiconductor substance formed by inertalumina or silica.
 6. The method for manufacturing an element formed bysemiconductor(s) according to claim 5, wherein the proportion of saidinert alumina or silica in said heterogeneous suspension is determinedbeforehand.
 7. The method for manufacturing an element formed bysemiconductor(s) according to claim 1, wherein said second semiconductorformed by said semiconductor organic substance is of the n and p type inorder to form n-p-n or p-n-p junctions.
 8. The method for manufacturingan element formed by semiconductor(s) according to claim 2 or 7, whereinthe proportion of type n, type p in said first and second semiconductorsubstances is determined beforehand.
 9. The method for manufacturing anelement formed by semiconductor(s) according to claim 1 wherein prior todispersing, said semiconductor substances are reduced in size bygrinding to less than 100 μm.
 10. The method for manufacturing anelement formed by semiconductor(s) according to claim 1, wherein afterdispersing said first and second semiconductor substances in thesolvent, the thus obtained heterogeneous suspension is submitted tointense agitation.
 11. The method for manufacturing an element formed bysemiconductor(s) according to claim 1, and further repeating steps (b)and (c) to form multiple layers.
 12. A combustion gas detector,comprising an element composed of semiconductor(s) obtained by themethod according to claim 1, which is serially mounted with anadjustment module and is connected to a verification unit provided fordetecting an amount of variation of resistivity value of said elementformed by semiconductor(s) and to generate an output signal afterdetection of a further amount of variation superior to a predeterminedthreshold.
 13. The gas detector according to claim 12, wherein saidverification unit comprises a microprocessor provided for producing anadjustment signal in response to a voltage change measured overterminals of said element formed by semiconductor(s).
 14. The gasdetector according to claim 13, wherein said adjustment module comprisesa transistor and a capacitor connected in parallel, said adjustmentsignals being supplied at the base of said transistor.